Overactive Then Underactive Thyroid — Why It Happens in That Order

The immune attack immunotherapy can trigger against the thyroid first inflames the gland, causing it to leak its own stored hormone into the blood — this produces the brief overactive (hyperthyroid) phase. As the inflammation continues, the gland's hormone-producing cells are affected and its own capacity to make new hormone falls, which is what produces the later, often longer-lasting underactive (hypothyroid) phase.

It helps to think of it less as two separate illnesses and more as one process seen at two different points — first a "leak" of hormone already stored in the gland, then a "shortfall" once that stock runs out and the gland's own production stays low. That is also why the order is consistent enough for your care team to expect it, even though not every patient goes through both phases the same way.

This page explains a common, manageable hormone pattern — not how to manage a hormone emergency yourself. See the escalation note above and the red-flag list below for when to call instead of reading on.

The overactive phase tends to bring a fast heartbeat, sweating, anxiety, tremor, or unplanned weight loss, while the underactive phase tends to bring fatigue, feeling unusually cold, weight gain, constipation, and dry skin. Both are gradual, developing over days to weeks — a routine TSH and free T4 blood test, done as part of your regular monitoring, confirms which phase you're in far more reliably than symptoms alone.

What this page is not describing is a hormone emergency. Watch for these signs, which point to something more serious than a routine thyroid shift and need emergency care instead:

For many patients who reach the underactive phase, yes — daily thyroid hormone replacement becomes a long-term, often lifelong routine, because the gland's own ability to make enough hormone does not usually return once it has been affected this way. The dose is set and adjusted only by your treating clinician, based on repeat TSH and free T4 blood tests taken over several weeks.

This is never something to start, stop, or change on your own, even once you feel well again. Some patients regain partial thyroid function over months, and in that case a clinician may lower the dose later — again only on their assessment, never on how you happen to feel on a given day.

In most cases, immunotherapy continues alongside thyroid monitoring and, where needed, hormone replacement — a two-phase thyroid pattern is not usually a reason to pause or stop your cancer treatment the way a gut, lung, or liver reaction can be. Thyroid function is checked as part of your routine pre-cycle blood work, so the pattern is usually caught by the numbers well before you would notice a symptom.

Our companion page on which hormone tests should be done and how often sets out the wider testing schedule your team follows, including thyroid, adrenal, and blood sugar checks together — since all three glands can be affected by the same immune process.

The thyroid is not the only gland immunotherapy can affect. If you notice unusual thirst, frequent urination, or blurred vision — symptoms that are easy to confuse with a thyroid change — that pattern points more toward new-onset diabetes than the thyroid. Our companion pages on new diabetes caused by immunotherapy and excessive thirst and urination on immunotherapy explain how that is picked up and managed, separately from the thyroid pattern described here.

Your care team tests for all of these together precisely because the early symptoms overlap — which is why a blood panel, not guesswork, decides which gland is actually involved.

For the full testing schedule behind everything on this page, see which hormone tests should be done and how often. If thirst, urination, or blood sugar are part of your picture, see new diabetes caused by immunotherapy and excessive thirst and urination on immunotherapy. For the wider picture of how immunotherapy is delivered and monitored at CION, see the immunotherapy overview.